Late Quaternary river terraces in the Central Mountain Range of Taiwan: A study of cover sediments across a terrace section along the Tachia River

摘要

The existence of fluvial terraces in the high mountains of Taiwan has been known for a long time already, but these terraces have not yet been studied in detail. This paper studies cover sediments across a terrace section in the Central Mountain Range located at the midstream reaches of the Tachia River at an elevation of 1430 m asl. Sediments were dated by means of optically stimulated luminescence and radiocarbon dating, yielding robust ages between 54.8 ± 7.0 ka and 0.42 ± 0.09 cal. ka BP. These dates are in good agreement with ages known for the laterite terraces (LT) in the mountain foreland. Deeply weathered reddish soils, comparable to those in the mountain forelands, are absent. Hence, in strati-graphic terms, the terraces in the high mountains cannot be correlated with those in the mountain foreland. Cover sediments comprise eolian sand and silt and various colluvial layers. Eolian sediments provide paleoenvironmental evidence of a relatively dry phase in early MIS 3 that coincides with the maximum extent of the glaciations in the Hsueh Shan Mountain area. Widespread colluvial sediments indicate a high frequency of mass movements around 38.5 ka, a period of time known to correlate with dominantly wet conditions. In the last 1000 years, wildfire appears as a possible trigger of mass movements. The geochronological data indicate a local bedrock strath incision rate of 0.8 ±0.1 ramy"1, a value that appears to be in agreement with the local decadal scale erosion rate.